Commit | Line | Data |
---|---|---|
e6512bcf RS |
1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
7baeca0c LT |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, |
4 | @c 2000, 2003, 2004 | |
177c0ea7 | 5 | @c Free Software Foundation, Inc. |
e6512bcf RS |
6 | @c See the file elisp.texi for copying conditions. |
7 | @setfilename ../info/variables | |
8 | @node Variables, Functions, Control Structures, Top | |
9 | @chapter Variables | |
10 | @cindex variable | |
11 | ||
12 | A @dfn{variable} is a name used in a program to stand for a value. | |
13 | Nearly all programming languages have variables of some sort. In the | |
14 | text of a Lisp program, variables are written using the syntax for | |
15 | symbols. | |
16 | ||
17 | In Lisp, unlike most programming languages, programs are represented | |
18 | primarily as Lisp objects and only secondarily as text. The Lisp | |
19 | objects used for variables are symbols: the symbol name is the variable | |
20 | name, and the variable's value is stored in the value cell of the | |
21 | symbol. The use of a symbol as a variable is independent of its use as | |
22 | a function name. @xref{Symbol Components}. | |
23 | ||
24 | The Lisp objects that constitute a Lisp program determine the textual | |
f57ddf67 | 25 | form of the program---it is simply the read syntax for those Lisp |
e6512bcf RS |
26 | objects. This is why, for example, a variable in a textual Lisp program |
27 | is written using the read syntax for the symbol that represents the | |
28 | variable. | |
29 | ||
30 | @menu | |
31 | * Global Variables:: Variable values that exist permanently, everywhere. | |
32 | * Constant Variables:: Certain "variables" have values that never change. | |
33 | * Local Variables:: Variable values that exist only temporarily. | |
34 | * Void Variables:: Symbols that lack values. | |
35 | * Defining Variables:: A definition says a symbol is used as a variable. | |
b50c9214 RS |
36 | * Tips for Defining:: Things you should think about when you |
37 | define a variable. | |
e6512bcf RS |
38 | * Accessing Variables:: Examining values of variables whose names |
39 | are known only at run time. | |
40 | * Setting Variables:: Storing new values in variables. | |
41 | * Variable Scoping:: How Lisp chooses among local and global values. | |
42 | * Buffer-Local Variables:: Variable values in effect only in one buffer. | |
969fe9b5 RS |
43 | * Frame-Local Variables:: Variable values in effect only in one frame. |
44 | * Future Local Variables:: New kinds of local values we might add some day. | |
26236f6d | 45 | * Variable Aliases:: Variables that are aliases for other variables. |
b50c9214 | 46 | * File Local Variables:: Handling local variable lists in files. |
74ab3aa3 LT |
47 | * Variables with Restricted Values:: Non-constant variables whose value can |
48 | @emph{not} be an arbitrary Lisp object. | |
e6512bcf RS |
49 | @end menu |
50 | ||
51 | @node Global Variables | |
52 | @section Global Variables | |
53 | @cindex global variable | |
54 | ||
55 | The simplest way to use a variable is @dfn{globally}. This means that | |
56 | the variable has just one value at a time, and this value is in effect | |
57 | (at least for the moment) throughout the Lisp system. The value remains | |
58 | in effect until you specify a new one. When a new value replaces the | |
59 | old one, no trace of the old value remains in the variable. | |
60 | ||
61 | You specify a value for a symbol with @code{setq}. For example, | |
62 | ||
63 | @example | |
64 | (setq x '(a b)) | |
65 | @end example | |
66 | ||
67 | @noindent | |
68 | gives the variable @code{x} the value @code{(a b)}. Note that | |
69 | @code{setq} does not evaluate its first argument, the name of the | |
70 | variable, but it does evaluate the second argument, the new value. | |
71 | ||
72 | Once the variable has a value, you can refer to it by using the symbol | |
73 | by itself as an expression. Thus, | |
74 | ||
75 | @example | |
76 | @group | |
77 | x @result{} (a b) | |
78 | @end group | |
79 | @end example | |
80 | ||
81 | @noindent | |
82 | assuming the @code{setq} form shown above has already been executed. | |
83 | ||
969fe9b5 RS |
84 | If you do set the same variable again, the new value replaces the old |
85 | one: | |
e6512bcf RS |
86 | |
87 | @example | |
88 | @group | |
89 | x | |
90 | @result{} (a b) | |
91 | @end group | |
92 | @group | |
93 | (setq x 4) | |
94 | @result{} 4 | |
95 | @end group | |
96 | @group | |
97 | x | |
98 | @result{} 4 | |
99 | @end group | |
100 | @end example | |
101 | ||
102 | @node Constant Variables | |
8241495d | 103 | @section Variables that Never Change |
e6512bcf RS |
104 | @vindex nil |
105 | @vindex t | |
106 | @kindex setting-constant | |
95ed62c7 | 107 | @cindex keyword symbol |
e6512bcf | 108 | |
969fe9b5 RS |
109 | In Emacs Lisp, certain symbols normally evaluate to themselves. These |
110 | include @code{nil} and @code{t}, as well as any symbol whose name starts | |
95ed62c7 GM |
111 | with @samp{:} (these are called @dfn{keywords}). These symbols cannot |
112 | be rebound, nor can their values be changed. Any attempt to set or bind | |
113 | @code{nil} or @code{t} signals a @code{setting-constant} error. The | |
114 | same is true for a keyword (a symbol whose name starts with @samp{:}), | |
115 | if it is interned in the standard obarray, except that setting such a | |
116 | symbol to itself is not an error. | |
e6512bcf RS |
117 | |
118 | @example | |
119 | @group | |
120 | nil @equiv{} 'nil | |
121 | @result{} nil | |
122 | @end group | |
123 | @group | |
124 | (setq nil 500) | |
125 | @error{} Attempt to set constant symbol: nil | |
126 | @end group | |
127 | @end example | |
128 | ||
e88399c8 DL |
129 | @defun keywordp object |
130 | @tindex keywordp | |
131 | function returns @code{t} if @var{object} is a symbol whose name | |
132 | starts with @samp{:}, interned in the standard obarray, and returns | |
133 | @code{nil} otherwise. | |
134 | @end defun | |
135 | ||
e6512bcf RS |
136 | @node Local Variables |
137 | @section Local Variables | |
138 | @cindex binding local variables | |
139 | @cindex local variables | |
140 | @cindex local binding | |
141 | @cindex global binding | |
142 | ||
143 | Global variables have values that last until explicitly superseded | |
144 | with new values. Sometimes it is useful to create variable values that | |
969fe9b5 | 145 | exist temporarily---only until a certain part of the program finishes. |
e6512bcf RS |
146 | These values are called @dfn{local}, and the variables so used are |
147 | called @dfn{local variables}. | |
148 | ||
149 | For example, when a function is called, its argument variables receive | |
150 | new local values that last until the function exits. The @code{let} | |
151 | special form explicitly establishes new local values for specified | |
152 | variables; these last until exit from the @code{let} form. | |
153 | ||
154 | @cindex shadowing of variables | |
155 | Establishing a local value saves away the previous value (or lack of | |
156 | one) of the variable. When the life span of the local value is over, | |
157 | the previous value is restored. In the mean time, we say that the | |
158 | previous value is @dfn{shadowed} and @dfn{not visible}. Both global and | |
159 | local values may be shadowed (@pxref{Scope}). | |
160 | ||
161 | If you set a variable (such as with @code{setq}) while it is local, | |
162 | this replaces the local value; it does not alter the global value, or | |
969fe9b5 | 163 | previous local values, that are shadowed. To model this behavior, we |
e6512bcf RS |
164 | speak of a @dfn{local binding} of the variable as well as a local value. |
165 | ||
166 | The local binding is a conceptual place that holds a local value. | |
167 | Entry to a function, or a special form such as @code{let}, creates the | |
168 | local binding; exit from the function or from the @code{let} removes the | |
169 | local binding. As long as the local binding lasts, the variable's value | |
170 | is stored within it. Use of @code{setq} or @code{set} while there is a | |
171 | local binding stores a different value into the local binding; it does | |
172 | not create a new binding. | |
173 | ||
174 | We also speak of the @dfn{global binding}, which is where | |
175 | (conceptually) the global value is kept. | |
176 | ||
177 | @cindex current binding | |
178 | A variable can have more than one local binding at a time (for | |
179 | example, if there are nested @code{let} forms that bind it). In such a | |
180 | case, the most recently created local binding that still exists is the | |
969fe9b5 RS |
181 | @dfn{current binding} of the variable. (This rule is called |
182 | @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no | |
183 | local bindings, the variable's global binding is its current binding. | |
184 | We sometimes call the current binding the @dfn{most-local existing | |
185 | binding}, for emphasis. Ordinary evaluation of a symbol always returns | |
186 | the value of its current binding. | |
e6512bcf RS |
187 | |
188 | The special forms @code{let} and @code{let*} exist to create | |
189 | local bindings. | |
190 | ||
191 | @defspec let (bindings@dots{}) forms@dots{} | |
f57ddf67 | 192 | This special form binds variables according to @var{bindings} and then |
e6512bcf RS |
193 | evaluates all of the @var{forms} in textual order. The @code{let}-form |
194 | returns the value of the last form in @var{forms}. | |
195 | ||
196 | Each of the @var{bindings} is either @w{(i) a} symbol, in which case | |
197 | that symbol is bound to @code{nil}; or @w{(ii) a} list of the form | |
198 | @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is | |
199 | bound to the result of evaluating @var{value-form}. If @var{value-form} | |
200 | is omitted, @code{nil} is used. | |
201 | ||
202 | All of the @var{value-form}s in @var{bindings} are evaluated in the | |
f9f59935 | 203 | order they appear and @emph{before} binding any of the symbols to them. |
5c646a2d RS |
204 | Here is an example of this: @code{z} is bound to the old value of |
205 | @code{y}, which is 2, not the new value of @code{y}, which is 1. | |
e6512bcf RS |
206 | |
207 | @example | |
208 | @group | |
5c646a2d | 209 | (setq y 2) |
e6512bcf RS |
210 | @result{} 2 |
211 | @end group | |
212 | @group | |
5c646a2d RS |
213 | (let ((y 1) |
214 | (z y)) | |
215 | (list y z)) | |
e6512bcf RS |
216 | @result{} (1 2) |
217 | @end group | |
218 | @end example | |
219 | @end defspec | |
220 | ||
221 | @defspec let* (bindings@dots{}) forms@dots{} | |
222 | This special form is like @code{let}, but it binds each variable right | |
223 | after computing its local value, before computing the local value for | |
224 | the next variable. Therefore, an expression in @var{bindings} can | |
225 | reasonably refer to the preceding symbols bound in this @code{let*} | |
226 | form. Compare the following example with the example above for | |
227 | @code{let}. | |
228 | ||
229 | @example | |
230 | @group | |
5c646a2d | 231 | (setq y 2) |
e6512bcf RS |
232 | @result{} 2 |
233 | @end group | |
234 | @group | |
5c646a2d RS |
235 | (let* ((y 1) |
236 | (z y)) ; @r{Use the just-established value of @code{y}.} | |
237 | (list y z)) | |
e6512bcf RS |
238 | @result{} (1 1) |
239 | @end group | |
240 | @end example | |
241 | @end defspec | |
242 | ||
f57ddf67 | 243 | Here is a complete list of the other facilities that create local |
e6512bcf RS |
244 | bindings: |
245 | ||
246 | @itemize @bullet | |
247 | @item | |
248 | Function calls (@pxref{Functions}). | |
249 | ||
250 | @item | |
251 | Macro calls (@pxref{Macros}). | |
252 | ||
253 | @item | |
254 | @code{condition-case} (@pxref{Errors}). | |
255 | @end itemize | |
256 | ||
bfe721d1 | 257 | Variables can also have buffer-local bindings (@pxref{Buffer-Local |
a9f0a989 RS |
258 | Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a |
259 | few variables have terminal-local bindings (@pxref{Multiple Displays}). | |
260 | These kinds of bindings work somewhat like ordinary local bindings, but | |
261 | they are localized depending on ``where'' you are in Emacs, rather than | |
262 | localized in time. | |
bfe721d1 | 263 | |
e6512bcf | 264 | @defvar max-specpdl-size |
7baeca0c | 265 | @anchor{Definition of max-specpdl-size} |
e6512bcf RS |
266 | @cindex variable limit error |
267 | @cindex evaluation error | |
268 | @cindex infinite recursion | |
a9f0a989 | 269 | This variable defines the limit on the total number of local variable |
74ab3aa3 LT |
270 | bindings and @code{unwind-protect} cleanups (@pxref{Cleanups,, |
271 | Cleaning Up from Nonlocal Exits}) that are allowed before signaling an | |
272 | error (with data @code{"Variable binding depth exceeds | |
273 | max-specpdl-size"}). | |
e6512bcf | 274 | |
a9f0a989 | 275 | This limit, with the associated error when it is exceeded, is one way |
e6512bcf | 276 | that Lisp avoids infinite recursion on an ill-defined function. |
a9f0a989 | 277 | @code{max-lisp-eval-depth} provides another limit on depth of nesting. |
74ab3aa3 | 278 | @xref{Definition of max-lisp-eval-depth,, Eval}. |
a9f0a989 RS |
279 | |
280 | The default value is 600. Entry to the Lisp debugger increases the | |
281 | value, if there is little room left, to make sure the debugger itself | |
282 | has room to execute. | |
e6512bcf RS |
283 | @end defvar |
284 | ||
285 | @node Void Variables | |
286 | @section When a Variable is ``Void'' | |
287 | @kindex void-variable | |
288 | @cindex void variable | |
289 | ||
290 | If you have never given a symbol any value as a global variable, we | |
291 | say that that symbol's global value is @dfn{void}. In other words, the | |
292 | symbol's value cell does not have any Lisp object in it. If you try to | |
293 | evaluate the symbol, you get a @code{void-variable} error rather than | |
294 | a value. | |
295 | ||
296 | Note that a value of @code{nil} is not the same as void. The symbol | |
297 | @code{nil} is a Lisp object and can be the value of a variable just as any | |
298 | other object can be; but it is @emph{a value}. A void variable does not | |
299 | have any value. | |
300 | ||
301 | After you have given a variable a value, you can make it void once more | |
302 | using @code{makunbound}. | |
303 | ||
304 | @defun makunbound symbol | |
f9f59935 | 305 | This function makes the current variable binding of @var{symbol} void. |
e6512bcf | 306 | Subsequent attempts to use this symbol's value as a variable will signal |
f9f59935 | 307 | the error @code{void-variable}, unless and until you set it again. |
e6512bcf RS |
308 | |
309 | @code{makunbound} returns @var{symbol}. | |
310 | ||
311 | @example | |
312 | @group | |
969fe9b5 | 313 | (makunbound 'x) ; @r{Make the global value of @code{x} void.} |
e6512bcf RS |
314 | @result{} x |
315 | @end group | |
316 | @group | |
317 | x | |
318 | @error{} Symbol's value as variable is void: x | |
319 | @end group | |
320 | @end example | |
321 | ||
322 | If @var{symbol} is locally bound, @code{makunbound} affects the most | |
323 | local existing binding. This is the only way a symbol can have a void | |
324 | local binding, since all the constructs that create local bindings | |
325 | create them with values. In this case, the voidness lasts at most as | |
326 | long as the binding does; when the binding is removed due to exit from | |
969fe9b5 RS |
327 | the construct that made it, the previous local or global binding is |
328 | reexposed as usual, and the variable is no longer void unless the newly | |
329 | reexposed binding was void all along. | |
e6512bcf RS |
330 | |
331 | @smallexample | |
332 | @group | |
333 | (setq x 1) ; @r{Put a value in the global binding.} | |
334 | @result{} 1 | |
335 | (let ((x 2)) ; @r{Locally bind it.} | |
336 | (makunbound 'x) ; @r{Void the local binding.} | |
337 | x) | |
338 | @error{} Symbol's value as variable is void: x | |
339 | @end group | |
340 | @group | |
341 | x ; @r{The global binding is unchanged.} | |
342 | @result{} 1 | |
343 | ||
344 | (let ((x 2)) ; @r{Locally bind it.} | |
345 | (let ((x 3)) ; @r{And again.} | |
346 | (makunbound 'x) ; @r{Void the innermost-local binding.} | |
347 | x)) ; @r{And refer: it's void.} | |
348 | @error{} Symbol's value as variable is void: x | |
349 | @end group | |
350 | ||
351 | @group | |
352 | (let ((x 2)) | |
353 | (let ((x 3)) | |
354 | (makunbound 'x)) ; @r{Void inner binding, then remove it.} | |
355 | x) ; @r{Now outer @code{let} binding is visible.} | |
356 | @result{} 2 | |
357 | @end group | |
358 | @end smallexample | |
359 | @end defun | |
360 | ||
361 | A variable that has been made void with @code{makunbound} is | |
362 | indistinguishable from one that has never received a value and has | |
363 | always been void. | |
364 | ||
365 | You can use the function @code{boundp} to test whether a variable is | |
366 | currently void. | |
367 | ||
368 | @defun boundp variable | |
369 | @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void; | |
370 | more precisely, if its current binding is not void. It returns | |
371 | @code{nil} otherwise. | |
372 | ||
373 | @smallexample | |
374 | @group | |
375 | (boundp 'abracadabra) ; @r{Starts out void.} | |
376 | @result{} nil | |
377 | @end group | |
378 | @group | |
379 | (let ((abracadabra 5)) ; @r{Locally bind it.} | |
380 | (boundp 'abracadabra)) | |
381 | @result{} t | |
382 | @end group | |
383 | @group | |
384 | (boundp 'abracadabra) ; @r{Still globally void.} | |
385 | @result{} nil | |
386 | @end group | |
387 | @group | |
388 | (setq abracadabra 5) ; @r{Make it globally nonvoid.} | |
389 | @result{} 5 | |
390 | @end group | |
391 | @group | |
392 | (boundp 'abracadabra) | |
393 | @result{} t | |
394 | @end group | |
395 | @end smallexample | |
396 | @end defun | |
397 | ||
398 | @node Defining Variables | |
399 | @section Defining Global Variables | |
f57ddf67 | 400 | @cindex variable definition |
e6512bcf RS |
401 | |
402 | You may announce your intention to use a symbol as a global variable | |
f57ddf67 RS |
403 | with a @dfn{variable definition}: a special form, either @code{defconst} |
404 | or @code{defvar}. | |
e6512bcf RS |
405 | |
406 | In Emacs Lisp, definitions serve three purposes. First, they inform | |
407 | people who read the code that certain symbols are @emph{intended} to be | |
408 | used a certain way (as variables). Second, they inform the Lisp system | |
409 | of these things, supplying a value and documentation. Third, they | |
410 | provide information to utilities such as @code{etags} and | |
411 | @code{make-docfile}, which create data bases of the functions and | |
412 | variables in a program. | |
413 | ||
414 | The difference between @code{defconst} and @code{defvar} is primarily | |
f9f59935 RS |
415 | a matter of intent, serving to inform human readers of whether the value |
416 | should ever change. Emacs Lisp does not restrict the ways in which a | |
417 | variable can be used based on @code{defconst} or @code{defvar} | |
f57ddf67 | 418 | declarations. However, it does make a difference for initialization: |
e6512bcf RS |
419 | @code{defconst} unconditionally initializes the variable, while |
420 | @code{defvar} initializes it only if it is void. | |
421 | ||
f9f59935 | 422 | @ignore |
e6512bcf RS |
423 | One would expect user option variables to be defined with |
424 | @code{defconst}, since programs do not change them. Unfortunately, this | |
425 | has bad results if the definition is in a library that is not preloaded: | |
426 | @code{defconst} would override any prior value when the library is | |
427 | loaded. Users would like to be able to set user options in their init | |
428 | files, and override the default values given in the definitions. For | |
429 | this reason, user options must be defined with @code{defvar}. | |
f9f59935 | 430 | @end ignore |
e6512bcf RS |
431 | |
432 | @defspec defvar symbol [value [doc-string]] | |
f9f59935 RS |
433 | This special form defines @var{symbol} as a variable and can also |
434 | initialize and document it. The definition informs a person reading | |
435 | your code that @var{symbol} is used as a variable that might be set or | |
436 | changed. Note that @var{symbol} is not evaluated; the symbol to be | |
e6512bcf RS |
437 | defined must appear explicitly in the @code{defvar}. |
438 | ||
f9f59935 RS |
439 | If @var{symbol} is void and @var{value} is specified, @code{defvar} |
440 | evaluates it and sets @var{symbol} to the result. But if @var{symbol} | |
441 | already has a value (i.e., it is not void), @var{value} is not even | |
442 | evaluated, and @var{symbol}'s value remains unchanged. If @var{value} | |
443 | is omitted, the value of @var{symbol} is not changed in any case. | |
e6512bcf | 444 | |
a9f0a989 RS |
445 | If @var{symbol} has a buffer-local binding in the current buffer, |
446 | @code{defvar} operates on the default value, which is buffer-independent, | |
447 | not the current (buffer-local) binding. It sets the default value if | |
448 | the default value is void. @xref{Buffer-Local Variables}. | |
449 | ||
bfe721d1 KH |
450 | When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in |
451 | Emacs Lisp mode (@code{eval-defun}), a special feature of | |
a9f0a989 RS |
452 | @code{eval-defun} arranges to set the variable unconditionally, without |
453 | testing whether its value is void. | |
e6512bcf RS |
454 | |
455 | If the @var{doc-string} argument appears, it specifies the documentation | |
456 | for the variable. (This opportunity to specify documentation is one of | |
457 | the main benefits of defining the variable.) The documentation is | |
458 | stored in the symbol's @code{variable-documentation} property. The | |
459 | Emacs help functions (@pxref{Documentation}) look for this property. | |
460 | ||
2a2048f2 RS |
461 | If the variable is a user option that users would want to set |
462 | interactively, you should use @samp{*} as the first character of | |
463 | @var{doc-string}. This lets users set the variable conveniently using | |
464 | the @code{set-variable} command. Note that you should nearly always | |
465 | use @code{defcustom} instead of @code{defvar} to define these | |
466 | variables, so that users can use @kbd{M-x customize} and related | |
467 | commands to set them. @xref{Customization}. | |
e6512bcf | 468 | |
f9f59935 RS |
469 | Here are some examples. This form defines @code{foo} but does not |
470 | initialize it: | |
e6512bcf RS |
471 | |
472 | @example | |
473 | @group | |
474 | (defvar foo) | |
475 | @result{} foo | |
476 | @end group | |
477 | @end example | |
478 | ||
f9f59935 RS |
479 | This example initializes the value of @code{bar} to @code{23}, and gives |
480 | it a documentation string: | |
e6512bcf RS |
481 | |
482 | @example | |
483 | @group | |
484 | (defvar bar 23 | |
485 | "The normal weight of a bar.") | |
486 | @result{} bar | |
487 | @end group | |
488 | @end example | |
489 | ||
490 | The following form changes the documentation string for @code{bar}, | |
491 | making it a user option, but does not change the value, since @code{bar} | |
f9f59935 RS |
492 | already has a value. (The addition @code{(1+ nil)} would get an error |
493 | if it were evaluated, but since it is not evaluated, there is no error.) | |
e6512bcf RS |
494 | |
495 | @example | |
496 | @group | |
f9f59935 | 497 | (defvar bar (1+ nil) |
e6512bcf RS |
498 | "*The normal weight of a bar.") |
499 | @result{} bar | |
500 | @end group | |
501 | @group | |
502 | bar | |
503 | @result{} 23 | |
504 | @end group | |
505 | @end example | |
506 | ||
507 | Here is an equivalent expression for the @code{defvar} special form: | |
508 | ||
509 | @example | |
510 | @group | |
511 | (defvar @var{symbol} @var{value} @var{doc-string}) | |
512 | @equiv{} | |
513 | (progn | |
514 | (if (not (boundp '@var{symbol})) | |
515 | (setq @var{symbol} @var{value})) | |
25ef19c5 RS |
516 | (if '@var{doc-string} |
517 | (put '@var{symbol} 'variable-documentation '@var{doc-string})) | |
e6512bcf RS |
518 | '@var{symbol}) |
519 | @end group | |
520 | @end example | |
521 | ||
522 | The @code{defvar} form returns @var{symbol}, but it is normally used | |
523 | at top level in a file where its value does not matter. | |
524 | @end defspec | |
525 | ||
f36e1798 | 526 | @defspec defconst symbol value [doc-string] |
e6512bcf | 527 | This special form defines @var{symbol} as a value and initializes it. |
f9f59935 RS |
528 | It informs a person reading your code that @var{symbol} has a standard |
529 | global value, established here, that should not be changed by the user | |
530 | or by other programs. Note that @var{symbol} is not evaluated; the | |
e6512bcf RS |
531 | symbol to be defined must appear explicitly in the @code{defconst}. |
532 | ||
f9f59935 | 533 | @code{defconst} always evaluates @var{value}, and sets the value of |
f36e1798 LT |
534 | @var{symbol} to the result. If @var{symbol} does have a buffer-local |
535 | binding in the current buffer, @code{defconst} sets the default value, | |
536 | not the buffer-local value. (But you should not be making | |
537 | buffer-local bindings for a symbol that is defined with | |
a9f0a989 | 538 | @code{defconst}.) |
e6512bcf RS |
539 | |
540 | Here, @code{pi} is a constant that presumably ought not to be changed | |
541 | by anyone (attempts by the Indiana State Legislature notwithstanding). | |
542 | As the second form illustrates, however, this is only advisory. | |
543 | ||
544 | @example | |
545 | @group | |
546 | (defconst pi 3.1415 "Pi to five places.") | |
547 | @result{} pi | |
548 | @end group | |
549 | @group | |
550 | (setq pi 3) | |
551 | @result{} pi | |
552 | @end group | |
553 | @group | |
554 | pi | |
555 | @result{} 3 | |
556 | @end group | |
557 | @end example | |
558 | @end defspec | |
559 | ||
560 | @defun user-variable-p variable | |
561 | @cindex user option | |
f57ddf67 | 562 | This function returns @code{t} if @var{variable} is a user option---a |
e6512bcf RS |
563 | variable intended to be set by the user for customization---and |
564 | @code{nil} otherwise. (Variables other than user options exist for the | |
565 | internal purposes of Lisp programs, and users need not know about them.) | |
566 | ||
e88399c8 DL |
567 | User option variables are distinguished from other variables either |
568 | though being declared using @code{defcustom}@footnote{They may also be | |
569 | declared equivalently in @file{cus-start.el}.} or by the first character | |
570 | of their @code{variable-documentation} property. If the property exists | |
571 | and is a string, and its first character is @samp{*}, then the variable | |
572 | is a user option. | |
e6512bcf RS |
573 | @end defun |
574 | ||
113613ea | 575 | @kindex variable-interactive |
e6512bcf | 576 | If a user option variable has a @code{variable-interactive} property, |
bfe721d1 KH |
577 | the @code{set-variable} command uses that value to control reading the |
578 | new value for the variable. The property's value is used as if it were | |
8241495d RS |
579 | specified in @code{interactive} (@pxref{Using Interactive}). However, |
580 | this feature is largely obsoleted by @code{defcustom} | |
581 | (@pxref{Customization}). | |
e6512bcf | 582 | |
b22f3a19 | 583 | @strong{Warning:} If the @code{defconst} and @code{defvar} special |
6fe50867 RS |
584 | forms are used while the variable has a local binding (made with |
585 | @code{let}, or a function argument), they set the local-binding's | |
586 | value; the top-level binding is not changed. This is not what you | |
587 | usually want. To prevent it, use these special forms at top level in | |
588 | a file, where normally no local binding is in effect, and make sure to | |
589 | load the file before making a local binding for the variable. | |
e6512bcf | 590 | |
aa9b77f6 RS |
591 | @node Tips for Defining |
592 | @section Tips for Defining Variables Robustly | |
593 | ||
b50c9214 RS |
594 | When you define a variable whose value is a function, or a list of |
595 | functions, use a name that ends in @samp{-function} or | |
596 | @samp{-functions}, respectively. | |
597 | ||
598 | There are several other variable name conventions; | |
599 | here is a complete list: | |
600 | ||
601 | @table @samp | |
602 | @item @dots{}-hook | |
603 | The variable is a normal hook (@pxref{Hooks}). | |
604 | ||
605 | @item @dots{}-function | |
606 | The value is a function. | |
607 | ||
608 | @item @dots{}-functions | |
609 | The value is a list of functions. | |
610 | ||
611 | @item @dots{}-form | |
612 | The value is a form (an expression). | |
613 | ||
80561aaa | 614 | @item @dots{}-forms |
b50c9214 RS |
615 | The value is a list of forms (expressions). |
616 | ||
617 | @item @dots{}-predicate | |
618 | The value is a predicate---a function of one argument that returns | |
619 | non-@code{nil} for ``good'' arguments and @code{nil} for ``bad'' | |
620 | arguments. | |
621 | ||
622 | @item @dots{}-flag | |
623 | The value is significant only as to whether it is @code{nil} or not. | |
624 | ||
625 | @item @dots{}-program | |
626 | The value is a program name. | |
627 | ||
628 | @item @dots{}-command | |
629 | The value is a whole shell command. | |
630 | ||
631 | @item @samp{}-switches | |
632 | The value specifies options for a command. | |
633 | @end table | |
634 | ||
80561aaa | 635 | When you define a variable, always consider whether you should mark |
b50c9214 RS |
636 | it as ``risky''; see @ref{File Local Variables}. |
637 | ||
aa9b77f6 RS |
638 | When defining and initializing a variable that holds a complicated |
639 | value (such as a keymap with bindings in it), it's best to put the | |
640 | entire computation of the value into the @code{defvar}, like this: | |
641 | ||
642 | @example | |
643 | (defvar my-mode-map | |
644 | (let ((map (make-sparse-keymap))) | |
f9f59935 | 645 | (define-key map "\C-c\C-a" 'my-command) |
aa9b77f6 RS |
646 | @dots{} |
647 | map) | |
648 | @var{docstring}) | |
649 | @end example | |
650 | ||
651 | @noindent | |
652 | This method has several benefits. First, if the user quits while | |
653 | loading the file, the variable is either still uninitialized or | |
969fe9b5 | 654 | initialized properly, never in-between. If it is still uninitialized, |
aa9b77f6 RS |
655 | reloading the file will initialize it properly. Second, reloading the |
656 | file once the variable is initialized will not alter it; that is | |
657 | important if the user has run hooks to alter part of the contents (such | |
658 | as, to rebind keys). Third, evaluating the @code{defvar} form with | |
659 | @kbd{C-M-x} @emph{will} reinitialize the map completely. | |
660 | ||
661 | Putting so much code in the @code{defvar} form has one disadvantage: | |
662 | it puts the documentation string far away from the line which names the | |
663 | variable. Here's a safe way to avoid that: | |
664 | ||
665 | @example | |
666 | (defvar my-mode-map nil | |
667 | @var{docstring}) | |
8241495d | 668 | (unless my-mode-map |
aa9b77f6 | 669 | (let ((map (make-sparse-keymap))) |
b548072f | 670 | (define-key map "\C-c\C-a" 'my-command) |
aa9b77f6 RS |
671 | @dots{} |
672 | (setq my-mode-map map))) | |
673 | @end example | |
674 | ||
675 | @noindent | |
676 | This has all the same advantages as putting the initialization inside | |
677 | the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on | |
678 | each form, if you do want to reinitialize the variable. | |
679 | ||
680 | But be careful not to write the code like this: | |
681 | ||
682 | @example | |
683 | (defvar my-mode-map nil | |
684 | @var{docstring}) | |
8241495d | 685 | (unless my-mode-map |
aa9b77f6 RS |
686 | (setq my-mode-map (make-sparse-keymap)) |
687 | (define-key my-mode-map "\C-c\C-a" 'my-command) | |
688 | @dots{}) | |
689 | @end example | |
690 | ||
691 | @noindent | |
969fe9b5 RS |
692 | This code sets the variable, then alters it, but it does so in more than |
693 | one step. If the user quits just after the @code{setq}, that leaves the | |
694 | variable neither correctly initialized nor void nor @code{nil}. Once | |
695 | that happens, reloading the file will not initialize the variable; it | |
696 | will remain incomplete. | |
aa9b77f6 | 697 | |
e6512bcf RS |
698 | @node Accessing Variables |
699 | @section Accessing Variable Values | |
700 | ||
701 | The usual way to reference a variable is to write the symbol which | |
702 | names it (@pxref{Symbol Forms}). This requires you to specify the | |
703 | variable name when you write the program. Usually that is exactly what | |
704 | you want to do. Occasionally you need to choose at run time which | |
705 | variable to reference; then you can use @code{symbol-value}. | |
706 | ||
707 | @defun symbol-value symbol | |
708 | This function returns the value of @var{symbol}. This is the value in | |
709 | the innermost local binding of the symbol, or its global value if it | |
710 | has no local bindings. | |
711 | ||
712 | @example | |
713 | @group | |
714 | (setq abracadabra 5) | |
715 | @result{} 5 | |
716 | @end group | |
717 | @group | |
718 | (setq foo 9) | |
719 | @result{} 9 | |
720 | @end group | |
721 | ||
722 | @group | |
723 | ;; @r{Here the symbol @code{abracadabra}} | |
724 | ;; @r{is the symbol whose value is examined.} | |
725 | (let ((abracadabra 'foo)) | |
726 | (symbol-value 'abracadabra)) | |
727 | @result{} foo | |
728 | @end group | |
729 | ||
730 | @group | |
731 | ;; @r{Here the value of @code{abracadabra},} | |
732 | ;; @r{which is @code{foo},} | |
733 | ;; @r{is the symbol whose value is examined.} | |
734 | (let ((abracadabra 'foo)) | |
735 | (symbol-value abracadabra)) | |
736 | @result{} 9 | |
737 | @end group | |
738 | ||
739 | @group | |
740 | (symbol-value 'abracadabra) | |
741 | @result{} 5 | |
742 | @end group | |
743 | @end example | |
744 | ||
969fe9b5 RS |
745 | A @code{void-variable} error is signaled if the current binding of |
746 | @var{symbol} is void. | |
e6512bcf RS |
747 | @end defun |
748 | ||
749 | @node Setting Variables | |
750 | @section How to Alter a Variable Value | |
751 | ||
752 | The usual way to change the value of a variable is with the special | |
753 | form @code{setq}. When you need to compute the choice of variable at | |
754 | run time, use the function @code{set}. | |
755 | ||
756 | @defspec setq [symbol form]@dots{} | |
757 | This special form is the most common method of changing a variable's | |
758 | value. Each @var{symbol} is given a new value, which is the result of | |
759 | evaluating the corresponding @var{form}. The most-local existing | |
760 | binding of the symbol is changed. | |
761 | ||
762 | @code{setq} does not evaluate @var{symbol}; it sets the symbol that you | |
763 | write. We say that this argument is @dfn{automatically quoted}. The | |
764 | @samp{q} in @code{setq} stands for ``quoted.'' | |
765 | ||
766 | The value of the @code{setq} form is the value of the last @var{form}. | |
767 | ||
768 | @example | |
769 | @group | |
770 | (setq x (1+ 2)) | |
771 | @result{} 3 | |
772 | @end group | |
773 | x ; @r{@code{x} now has a global value.} | |
774 | @result{} 3 | |
775 | @group | |
177c0ea7 | 776 | (let ((x 5)) |
e6512bcf RS |
777 | (setq x 6) ; @r{The local binding of @code{x} is set.} |
778 | x) | |
779 | @result{} 6 | |
780 | @end group | |
781 | x ; @r{The global value is unchanged.} | |
782 | @result{} 3 | |
783 | @end example | |
784 | ||
785 | Note that the first @var{form} is evaluated, then the first | |
786 | @var{symbol} is set, then the second @var{form} is evaluated, then the | |
787 | second @var{symbol} is set, and so on: | |
788 | ||
789 | @example | |
790 | @group | |
791 | (setq x 10 ; @r{Notice that @code{x} is set before} | |
792 | y (1+ x)) ; @r{the value of @code{y} is computed.} | |
177c0ea7 | 793 | @result{} 11 |
e6512bcf RS |
794 | @end group |
795 | @end example | |
796 | @end defspec | |
797 | ||
798 | @defun set symbol value | |
799 | This function sets @var{symbol}'s value to @var{value}, then returns | |
800 | @var{value}. Since @code{set} is a function, the expression written for | |
801 | @var{symbol} is evaluated to obtain the symbol to set. | |
802 | ||
803 | The most-local existing binding of the variable is the binding that is | |
f57ddf67 | 804 | set; shadowed bindings are not affected. |
e6512bcf RS |
805 | |
806 | @example | |
807 | @group | |
808 | (set one 1) | |
809 | @error{} Symbol's value as variable is void: one | |
810 | @end group | |
811 | @group | |
812 | (set 'one 1) | |
813 | @result{} 1 | |
814 | @end group | |
815 | @group | |
816 | (set 'two 'one) | |
817 | @result{} one | |
818 | @end group | |
819 | @group | |
820 | (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} | |
821 | @result{} 2 | |
822 | @end group | |
823 | @group | |
824 | one ; @r{So it is @code{one} that was set.} | |
825 | @result{} 2 | |
826 | (let ((one 1)) ; @r{This binding of @code{one} is set,} | |
827 | (set 'one 3) ; @r{not the global value.} | |
828 | one) | |
829 | @result{} 3 | |
830 | @end group | |
831 | @group | |
832 | one | |
833 | @result{} 2 | |
834 | @end group | |
835 | @end example | |
836 | ||
f57ddf67 RS |
837 | If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} |
838 | error is signaled. | |
839 | ||
840 | @example | |
841 | (set '(x y) 'z) | |
842 | @error{} Wrong type argument: symbolp, (x y) | |
843 | @end example | |
844 | ||
e6512bcf RS |
845 | Logically speaking, @code{set} is a more fundamental primitive than |
846 | @code{setq}. Any use of @code{setq} can be trivially rewritten to use | |
847 | @code{set}; @code{setq} could even be defined as a macro, given the | |
848 | availability of @code{set}. However, @code{set} itself is rarely used; | |
f57ddf67 RS |
849 | beginners hardly need to know about it. It is useful only for choosing |
850 | at run time which variable to set. For example, the command | |
e6512bcf RS |
851 | @code{set-variable}, which reads a variable name from the user and then |
852 | sets the variable, needs to use @code{set}. | |
853 | ||
854 | @cindex CL note---@code{set} local | |
855 | @quotation | |
f57ddf67 | 856 | @b{Common Lisp note:} In Common Lisp, @code{set} always changes the |
f9f59935 RS |
857 | symbol's ``special'' or dynamic value, ignoring any lexical bindings. |
858 | In Emacs Lisp, all variables and all bindings are dynamic, so @code{set} | |
e6512bcf RS |
859 | always affects the most local existing binding. |
860 | @end quotation | |
861 | @end defun | |
862 | ||
22697dac KH |
863 | One other function for setting a variable is designed to add |
864 | an element to a list if it is not already present in the list. | |
865 | ||
f36e1798 | 866 | @defun add-to-list symbol element &optional append |
22697dac KH |
867 | This function sets the variable @var{symbol} by consing @var{element} |
868 | onto the old value, if @var{element} is not already a member of that | |
bfe721d1 KH |
869 | value. It returns the resulting list, whether updated or not. The |
870 | value of @var{symbol} had better be a list already before the call. | |
f36e1798 LT |
871 | Membership is tested using @code{equal}. |
872 | ||
873 | Normally, if @var{element} is added, it is added to the front of | |
874 | @var{symbol}, but if the optional argument @var{append} is | |
875 | non-@code{nil}, it is added at the end. | |
bfe721d1 KH |
876 | |
877 | The argument @var{symbol} is not implicitly quoted; @code{add-to-list} | |
878 | is an ordinary function, like @code{set} and unlike @code{setq}. Quote | |
879 | the argument yourself if that is what you want. | |
969fe9b5 | 880 | @end defun |
22697dac KH |
881 | |
882 | Here's a scenario showing how to use @code{add-to-list}: | |
883 | ||
884 | @example | |
885 | (setq foo '(a b)) | |
886 | @result{} (a b) | |
887 | ||
888 | (add-to-list 'foo 'c) ;; @r{Add @code{c}.} | |
889 | @result{} (c a b) | |
890 | ||
891 | (add-to-list 'foo 'b) ;; @r{No effect.} | |
892 | @result{} (c a b) | |
893 | ||
894 | foo ;; @r{@code{foo} was changed.} | |
895 | @result{} (c a b) | |
896 | @end example | |
22697dac KH |
897 | |
898 | An equivalent expression for @code{(add-to-list '@var{var} | |
899 | @var{value})} is this: | |
900 | ||
901 | @example | |
902 | (or (member @var{value} @var{var}) | |
903 | (setq @var{var} (cons @var{value} @var{var}))) | |
904 | @end example | |
905 | ||
e6512bcf RS |
906 | @node Variable Scoping |
907 | @section Scoping Rules for Variable Bindings | |
908 | ||
f9f59935 | 909 | A given symbol @code{foo} can have several local variable bindings, |
e6512bcf RS |
910 | established at different places in the Lisp program, as well as a global |
911 | binding. The most recently established binding takes precedence over | |
912 | the others. | |
913 | ||
914 | @cindex scope | |
915 | @cindex extent | |
916 | @cindex dynamic scoping | |
04b6b342 | 917 | @cindex lexical scoping |
e6512bcf RS |
918 | Local bindings in Emacs Lisp have @dfn{indefinite scope} and |
919 | @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in | |
8241495d | 920 | the source code the binding can be accessed. ``Indefinite scope'' means |
e6512bcf RS |
921 | that any part of the program can potentially access the variable |
922 | binding. @dfn{Extent} refers to @emph{when}, as the program is | |
8241495d | 923 | executing, the binding exists. ``Dynamic extent'' means that the binding |
e6512bcf RS |
924 | lasts as long as the activation of the construct that established it. |
925 | ||
926 | The combination of dynamic extent and indefinite scope is called | |
927 | @dfn{dynamic scoping}. By contrast, most programming languages use | |
928 | @dfn{lexical scoping}, in which references to a local variable must be | |
929 | located textually within the function or block that binds the variable. | |
930 | ||
931 | @cindex CL note---special variables | |
932 | @quotation | |
f9f59935 RS |
933 | @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are |
934 | dynamically scoped, like all variables in Emacs Lisp. | |
e6512bcf RS |
935 | @end quotation |
936 | ||
937 | @menu | |
938 | * Scope:: Scope means where in the program a value is visible. | |
939 | Comparison with other languages. | |
940 | * Extent:: Extent means how long in time a value exists. | |
941 | * Impl of Scope:: Two ways to implement dynamic scoping. | |
942 | * Using Scoping:: How to use dynamic scoping carefully and avoid problems. | |
943 | @end menu | |
944 | ||
945 | @node Scope | |
946 | @subsection Scope | |
947 | ||
948 | Emacs Lisp uses @dfn{indefinite scope} for local variable bindings. | |
949 | This means that any function anywhere in the program text might access a | |
950 | given binding of a variable. Consider the following function | |
951 | definitions: | |
952 | ||
953 | @example | |
954 | @group | |
955 | (defun binder (x) ; @r{@code{x} is bound in @code{binder}.} | |
956 | (foo 5)) ; @r{@code{foo} is some other function.} | |
957 | @end group | |
958 | ||
959 | @group | |
f9f59935 | 960 | (defun user () ; @r{@code{x} is used ``free'' in @code{user}.} |
e6512bcf RS |
961 | (list x)) |
962 | @end group | |
963 | @end example | |
964 | ||
965 | In a lexically scoped language, the binding of @code{x} in | |
966 | @code{binder} would never be accessible in @code{user}, because | |
967 | @code{user} is not textually contained within the function | |
8241495d | 968 | @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user} |
e6512bcf | 969 | may or may not refer to the binding of @code{x} established in |
8241495d | 970 | @code{binder}, depending on the circumstances: |
e6512bcf RS |
971 | |
972 | @itemize @bullet | |
973 | @item | |
974 | If we call @code{user} directly without calling @code{binder} at all, | |
975 | then whatever binding of @code{x} is found, it cannot come from | |
976 | @code{binder}. | |
977 | ||
978 | @item | |
f9f59935 | 979 | If we define @code{foo} as follows and then call @code{binder}, then the |
e6512bcf RS |
980 | binding made in @code{binder} will be seen in @code{user}: |
981 | ||
982 | @example | |
983 | @group | |
984 | (defun foo (lose) | |
985 | (user)) | |
986 | @end group | |
987 | @end example | |
988 | ||
989 | @item | |
f9f59935 RS |
990 | However, if we define @code{foo} as follows and then call @code{binder}, |
991 | then the binding made in @code{binder} @emph{will not} be seen in | |
992 | @code{user}: | |
e6512bcf RS |
993 | |
994 | @example | |
995 | (defun foo (x) | |
996 | (user)) | |
997 | @end example | |
998 | ||
999 | @noindent | |
1000 | Here, when @code{foo} is called by @code{binder}, it binds @code{x}. | |
1001 | (The binding in @code{foo} is said to @dfn{shadow} the one made in | |
1002 | @code{binder}.) Therefore, @code{user} will access the @code{x} bound | |
1003 | by @code{foo} instead of the one bound by @code{binder}. | |
1004 | @end itemize | |
1005 | ||
f9f59935 RS |
1006 | Emacs Lisp uses dynamic scoping because simple implementations of |
1007 | lexical scoping are slow. In addition, every Lisp system needs to offer | |
1008 | dynamic scoping at least as an option; if lexical scoping is the norm, | |
1009 | there must be a way to specify dynamic scoping instead for a particular | |
1010 | variable. It might not be a bad thing for Emacs to offer both, but | |
1011 | implementing it with dynamic scoping only was much easier. | |
1012 | ||
e6512bcf RS |
1013 | @node Extent |
1014 | @subsection Extent | |
1015 | ||
1016 | @dfn{Extent} refers to the time during program execution that a | |
1017 | variable name is valid. In Emacs Lisp, a variable is valid only while | |
1018 | the form that bound it is executing. This is called @dfn{dynamic | |
1019 | extent}. ``Local'' or ``automatic'' variables in most languages, | |
1020 | including C and Pascal, have dynamic extent. | |
1021 | ||
1022 | One alternative to dynamic extent is @dfn{indefinite extent}. This | |
1023 | means that a variable binding can live on past the exit from the form | |
1024 | that made the binding. Common Lisp and Scheme, for example, support | |
1025 | this, but Emacs Lisp does not. | |
1026 | ||
1027 | To illustrate this, the function below, @code{make-add}, returns a | |
f9f59935 RS |
1028 | function that purports to add @var{n} to its own argument @var{m}. This |
1029 | would work in Common Lisp, but it does not do the job in Emacs Lisp, | |
1030 | because after the call to @code{make-add} exits, the variable @code{n} | |
1031 | is no longer bound to the actual argument 2. | |
e6512bcf RS |
1032 | |
1033 | @example | |
1034 | (defun make-add (n) | |
1035 | (function (lambda (m) (+ n m)))) ; @r{Return a function.} | |
1036 | @result{} make-add | |
1037 | (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}} | |
1038 | ; @r{with @code{(make-add 2)}.} | |
1039 | @result{} (lambda (m) (+ n m)) | |
1040 | (add2 4) ; @r{Try to add 2 to 4.} | |
1041 | @error{} Symbol's value as variable is void: n | |
1042 | @end example | |
1043 | ||
1044 | @cindex closures not available | |
1045 | Some Lisp dialects have ``closures'', objects that are like functions | |
1046 | but record additional variable bindings. Emacs Lisp does not have | |
1047 | closures. | |
1048 | ||
1049 | @node Impl of Scope | |
1050 | @subsection Implementation of Dynamic Scoping | |
1051 | @cindex deep binding | |
1052 | ||
1053 | A simple sample implementation (which is not how Emacs Lisp actually | |
1054 | works) may help you understand dynamic binding. This technique is | |
1055 | called @dfn{deep binding} and was used in early Lisp systems. | |
1056 | ||
969fe9b5 RS |
1057 | Suppose there is a stack of bindings, which are variable-value pairs. |
1058 | At entry to a function or to a @code{let} form, we can push bindings | |
1059 | onto the stack for the arguments or local variables created there. We | |
1060 | can pop those bindings from the stack at exit from the binding | |
1061 | construct. | |
e6512bcf RS |
1062 | |
1063 | We can find the value of a variable by searching the stack from top to | |
1064 | bottom for a binding for that variable; the value from that binding is | |
1065 | the value of the variable. To set the variable, we search for the | |
1066 | current binding, then store the new value into that binding. | |
1067 | ||
1068 | As you can see, a function's bindings remain in effect as long as it | |
1069 | continues execution, even during its calls to other functions. That is | |
1070 | why we say the extent of the binding is dynamic. And any other function | |
1071 | can refer to the bindings, if it uses the same variables while the | |
1072 | bindings are in effect. That is why we say the scope is indefinite. | |
1073 | ||
1074 | @cindex shallow binding | |
1075 | The actual implementation of variable scoping in GNU Emacs Lisp uses a | |
1076 | technique called @dfn{shallow binding}. Each variable has a standard | |
1077 | place in which its current value is always found---the value cell of the | |
1078 | symbol. | |
1079 | ||
1080 | In shallow binding, setting the variable works by storing a value in | |
1081 | the value cell. Creating a new binding works by pushing the old value | |
f9f59935 RS |
1082 | (belonging to a previous binding) onto a stack, and storing the new |
1083 | local value in the value cell. Eliminating a binding works by popping | |
1084 | the old value off the stack, into the value cell. | |
e6512bcf RS |
1085 | |
1086 | We use shallow binding because it has the same results as deep | |
1087 | binding, but runs faster, since there is never a need to search for a | |
1088 | binding. | |
1089 | ||
1090 | @node Using Scoping | |
1091 | @subsection Proper Use of Dynamic Scoping | |
1092 | ||
1093 | Binding a variable in one function and using it in another is a | |
1094 | powerful technique, but if used without restraint, it can make programs | |
1095 | hard to understand. There are two clean ways to use this technique: | |
1096 | ||
1097 | @itemize @bullet | |
1098 | @item | |
1099 | Use or bind the variable only in a few related functions, written close | |
1100 | together in one file. Such a variable is used for communication within | |
1101 | one program. | |
1102 | ||
1103 | You should write comments to inform other programmers that they can see | |
1104 | all uses of the variable before them, and to advise them not to add uses | |
1105 | elsewhere. | |
1106 | ||
1107 | @item | |
1108 | Give the variable a well-defined, documented meaning, and make all | |
1109 | appropriate functions refer to it (but not bind it or set it) wherever | |
1110 | that meaning is relevant. For example, the variable | |
1111 | @code{case-fold-search} is defined as ``non-@code{nil} means ignore case | |
1112 | when searching''; various search and replace functions refer to it | |
1113 | directly or through their subroutines, but do not bind or set it. | |
1114 | ||
1115 | Then you can bind the variable in other programs, knowing reliably what | |
1116 | the effect will be. | |
1117 | @end itemize | |
1118 | ||
bfe721d1 KH |
1119 | In either case, you should define the variable with @code{defvar}. |
1120 | This helps other people understand your program by telling them to look | |
1121 | for inter-function usage. It also avoids a warning from the byte | |
1122 | compiler. Choose the variable's name to avoid name conflicts---don't | |
1123 | use short names like @code{x}. | |
1124 | ||
e6512bcf RS |
1125 | @node Buffer-Local Variables |
1126 | @section Buffer-Local Variables | |
1127 | @cindex variables, buffer-local | |
1128 | @cindex buffer-local variables | |
1129 | ||
1130 | Global and local variable bindings are found in most programming | |
8241495d | 1131 | languages in one form or another. Emacs, however, also supports additional, |
969fe9b5 | 1132 | unusual kinds of variable binding: @dfn{buffer-local} bindings, which |
8241495d | 1133 | apply only in one buffer, and @dfn{frame-local} bindings, which apply only in |
969fe9b5 RS |
1134 | one frame. Having different values for a variable in different buffers |
1135 | and/or frames is an important customization method. | |
1136 | ||
1137 | This section describes buffer-local bindings; for frame-local | |
1138 | bindings, see the following section, @ref{Frame-Local Variables}. (A few | |
1911e6e5 | 1139 | variables have bindings that are local to each terminal; see |
969fe9b5 | 1140 | @ref{Multiple Displays}.) |
e6512bcf RS |
1141 | |
1142 | @menu | |
1143 | * Intro to Buffer-Local:: Introduction and concepts. | |
1144 | * Creating Buffer-Local:: Creating and destroying buffer-local bindings. | |
1145 | * Default Value:: The default value is seen in buffers | |
969fe9b5 | 1146 | that don't have their own buffer-local values. |
e6512bcf RS |
1147 | @end menu |
1148 | ||
1149 | @node Intro to Buffer-Local | |
1150 | @subsection Introduction to Buffer-Local Variables | |
1151 | ||
1152 | A buffer-local variable has a buffer-local binding associated with a | |
1153 | particular buffer. The binding is in effect when that buffer is | |
1154 | current; otherwise, it is not in effect. If you set the variable while | |
1155 | a buffer-local binding is in effect, the new value goes in that binding, | |
f9f59935 RS |
1156 | so its other bindings are unchanged. This means that the change is |
1157 | visible only in the buffer where you made it. | |
1158 | ||
1159 | The variable's ordinary binding, which is not associated with any | |
1160 | specific buffer, is called the @dfn{default binding}. In most cases, | |
1161 | this is the global binding. | |
e6512bcf | 1162 | |
f9f59935 RS |
1163 | A variable can have buffer-local bindings in some buffers but not in |
1164 | other buffers. The default binding is shared by all the buffers that | |
969fe9b5 | 1165 | don't have their own bindings for the variable. (This includes all |
8241495d | 1166 | newly-created buffers.) If you set the variable in a buffer that does |
969fe9b5 RS |
1167 | not have a buffer-local binding for it, this sets the default binding |
1168 | (assuming there are no frame-local bindings to complicate the matter), | |
1169 | so the new value is visible in all the buffers that see the default | |
1170 | binding. | |
e6512bcf RS |
1171 | |
1172 | The most common use of buffer-local bindings is for major modes to change | |
1173 | variables that control the behavior of commands. For example, C mode and | |
1174 | Lisp mode both set the variable @code{paragraph-start} to specify that only | |
1175 | blank lines separate paragraphs. They do this by making the variable | |
1176 | buffer-local in the buffer that is being put into C mode or Lisp mode, and | |
969fe9b5 | 1177 | then setting it to the new value for that mode. @xref{Major Modes}. |
e6512bcf RS |
1178 | |
1179 | The usual way to make a buffer-local binding is with | |
969fe9b5 RS |
1180 | @code{make-local-variable}, which is what major mode commands typically |
1181 | use. This affects just the current buffer; all other buffers (including | |
1182 | those yet to be created) will continue to share the default value unless | |
1183 | they are explicitly given their own buffer-local bindings. | |
e6512bcf RS |
1184 | |
1185 | @cindex automatically buffer-local | |
1186 | A more powerful operation is to mark the variable as | |
1187 | @dfn{automatically buffer-local} by calling | |
1188 | @code{make-variable-buffer-local}. You can think of this as making the | |
1189 | variable local in all buffers, even those yet to be created. More | |
1190 | precisely, the effect is that setting the variable automatically makes | |
1191 | the variable local to the current buffer if it is not already so. All | |
f9f59935 | 1192 | buffers start out by sharing the default value of the variable as usual, |
969fe9b5 | 1193 | but setting the variable creates a buffer-local binding for the current |
e6512bcf | 1194 | buffer. The new value is stored in the buffer-local binding, leaving |
969fe9b5 RS |
1195 | the default binding untouched. This means that the default value cannot |
1196 | be changed with @code{setq} in any buffer; the only way to change it is | |
1197 | with @code{setq-default}. | |
f9f59935 | 1198 | |
beab64dc RS |
1199 | @strong{Warning:} When a variable has buffer-local or frame-local |
1200 | bindings in one or more buffers, @code{let} rebinds the binding that's | |
1201 | currently in effect. For instance, if the current buffer has a | |
1202 | buffer-local value, @code{let} temporarily rebinds that. If no | |
1203 | buffer-local or frame-local bindings are in effect, @code{let} rebinds | |
1204 | the default value. If inside the @code{let} you then change to a | |
1205 | different current buffer in which a different binding is in effect, | |
1206 | you won't see the @code{let} binding any more. And if you exit the | |
1207 | @code{let} while still in the other buffer, you won't see the | |
1208 | unbinding occur (though it will occur properly). Here is an example | |
1209 | to illustrate: | |
e6512bcf RS |
1210 | |
1211 | @example | |
1212 | @group | |
c152cd2a | 1213 | (setq foo 'g) |
e6512bcf RS |
1214 | (set-buffer "a") |
1215 | (make-local-variable 'foo) | |
1216 | @end group | |
1217 | (setq foo 'a) | |
1218 | (let ((foo 'temp)) | |
c152cd2a | 1219 | ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}} |
e6512bcf | 1220 | (set-buffer "b") |
c152cd2a | 1221 | ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}} |
bfe721d1 | 1222 | @var{body}@dots{}) |
e6512bcf | 1223 | @group |
beab64dc RS |
1224 | foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},} |
1225 | ; @r{but we don't see that in buffer @samp{b}} | |
e6512bcf RS |
1226 | @end group |
1227 | @group | |
beab64dc RS |
1228 | (set-buffer "a") ; @r{verify the local value was restored} |
1229 | foo @result{} 'a | |
e6512bcf RS |
1230 | @end group |
1231 | @end example | |
1232 | ||
1233 | Note that references to @code{foo} in @var{body} access the | |
1234 | buffer-local binding of buffer @samp{b}. | |
1235 | ||
1236 | When a file specifies local variable values, these become buffer-local | |
969fe9b5 RS |
1237 | values when you visit the file. @xref{File Variables,,, emacs, The |
1238 | GNU Emacs Manual}. | |
e6512bcf RS |
1239 | |
1240 | @node Creating Buffer-Local | |
1241 | @subsection Creating and Deleting Buffer-Local Bindings | |
1242 | ||
1243 | @deffn Command make-local-variable variable | |
1244 | This function creates a buffer-local binding in the current buffer for | |
1245 | @var{variable} (a symbol). Other buffers are not affected. The value | |
1246 | returned is @var{variable}. | |
1247 | ||
1248 | @c Emacs 19 feature | |
1249 | The buffer-local value of @var{variable} starts out as the same value | |
1250 | @var{variable} previously had. If @var{variable} was void, it remains | |
1251 | void. | |
1252 | ||
1253 | @example | |
1254 | @group | |
1255 | ;; @r{In buffer @samp{b1}:} | |
1256 | (setq foo 5) ; @r{Affects all buffers.} | |
1257 | @result{} 5 | |
1258 | @end group | |
1259 | @group | |
1260 | (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} | |
1261 | @result{} foo | |
1262 | @end group | |
1263 | @group | |
1264 | foo ; @r{That did not change} | |
1265 | @result{} 5 ; @r{the value.} | |
1266 | @end group | |
1267 | @group | |
1268 | (setq foo 6) ; @r{Change the value} | |
1269 | @result{} 6 ; @r{in @samp{b1}.} | |
1270 | @end group | |
1271 | @group | |
1272 | foo | |
1273 | @result{} 6 | |
1274 | @end group | |
1275 | ||
1276 | @group | |
1277 | ;; @r{In buffer @samp{b2}, the value hasn't changed.} | |
1278 | (save-excursion | |
1279 | (set-buffer "b2") | |
1280 | foo) | |
1281 | @result{} 5 | |
1282 | @end group | |
1283 | @end example | |
e8505179 RS |
1284 | |
1285 | Making a variable buffer-local within a @code{let}-binding for that | |
969fe9b5 RS |
1286 | variable does not work reliably, unless the buffer in which you do this |
1287 | is not current either on entry to or exit from the @code{let}. This is | |
1288 | because @code{let} does not distinguish between different kinds of | |
1289 | bindings; it knows only which variable the binding was made for. | |
22697dac | 1290 | |
bfe721d1 KH |
1291 | If the variable is terminal-local, this function signals an error. Such |
1292 | variables cannot have buffer-local bindings as well. @xref{Multiple | |
1293 | Displays}. | |
1294 | ||
6142d1d0 RS |
1295 | @strong{Warning:} do not use @code{make-local-variable} for a hook |
1296 | variable. The hook variables are automatically made buffer-local as | |
1297 | needed if you use the @var{local} argument to @code{add-hook} or | |
89cda0c5 | 1298 | @code{remove-hook}. |
e6512bcf RS |
1299 | @end deffn |
1300 | ||
1301 | @deffn Command make-variable-buffer-local variable | |
1302 | This function marks @var{variable} (a symbol) automatically | |
1303 | buffer-local, so that any subsequent attempt to set it will make it | |
1304 | local to the current buffer at the time. | |
1305 | ||
969fe9b5 RS |
1306 | A peculiar wrinkle of this feature is that binding the variable (with |
1307 | @code{let} or other binding constructs) does not create a buffer-local | |
1308 | binding for it. Only setting the variable (with @code{set} or | |
f36e1798 LT |
1309 | @code{setq}), while the variable does not have a @code{let}-style |
1310 | binding that was made in the current buffer, does so. | |
1311 | ||
1312 | If @var{variable} does not have a default value, then calling this | |
1313 | command will give it a default value of @code{nil}. If @var{variable} | |
1314 | already has a default value, that value remains unchanged. | |
1315 | Subsequently calling @code{makunbound} on @var{variable} will result | |
1316 | in a void buffer-local value and leave the default value unaffected. | |
ab4b1835 | 1317 | |
969fe9b5 | 1318 | The value returned is @var{variable}. |
ab4b1835 | 1319 | |
969fe9b5 RS |
1320 | @strong{Warning:} Don't assume that you should use |
1321 | @code{make-variable-buffer-local} for user-option variables, simply | |
1322 | because users @emph{might} want to customize them differently in | |
1323 | different buffers. Users can make any variable local, when they wish | |
1324 | to. It is better to leave the choice to them. | |
1325 | ||
1326 | The time to use @code{make-variable-buffer-local} is when it is crucial | |
1327 | that no two buffers ever share the same binding. For example, when a | |
1328 | variable is used for internal purposes in a Lisp program which depends | |
1329 | on having separate values in separate buffers, then using | |
1330 | @code{make-variable-buffer-local} can be the best solution. | |
e6512bcf RS |
1331 | @end deffn |
1332 | ||
bfe721d1 KH |
1333 | @defun local-variable-p variable &optional buffer |
1334 | This returns @code{t} if @var{variable} is buffer-local in buffer | |
1335 | @var{buffer} (which defaults to the current buffer); otherwise, | |
1336 | @code{nil}. | |
1337 | @end defun | |
1338 | ||
dbcd9cd7 RS |
1339 | @defun local-variable-if-set-p variable &optional buffer |
1340 | This returns @code{t} if @var{variable} will become buffer-local in | |
1341 | buffer @var{buffer} (which defaults to the current buffer) if it is | |
1342 | set there. | |
1343 | @end defun | |
1344 | ||
a1c07461 | 1345 | @defun buffer-local-value variable buffer |
2b99c69a RS |
1346 | This function returns the buffer-local binding of @var{variable} (a |
1347 | symbol) in buffer @var{buffer}. If @var{variable} does not have a | |
1348 | buffer-local binding in buffer @var{buffer}, it returns the default | |
1349 | value (@pxref{Default Value}) of @var{variable} instead. | |
a1c07461 RS |
1350 | @end defun |
1351 | ||
e6512bcf RS |
1352 | @defun buffer-local-variables &optional buffer |
1353 | This function returns a list describing the buffer-local variables in | |
969fe9b5 RS |
1354 | buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is |
1355 | used.) It returns an association list (@pxref{Association Lists}) in | |
1356 | which each element contains one buffer-local variable and its value. | |
1357 | However, when a variable's buffer-local binding in @var{buffer} is void, | |
1358 | then the variable appears directly in the resulting list. | |
e6512bcf RS |
1359 | |
1360 | @example | |
1361 | @group | |
1362 | (make-local-variable 'foobar) | |
1363 | (makunbound 'foobar) | |
1364 | (make-local-variable 'bind-me) | |
1365 | (setq bind-me 69) | |
1366 | @end group | |
1367 | (setq lcl (buffer-local-variables)) | |
1368 | ;; @r{First, built-in variables local in all buffers:} | |
1369 | @result{} ((mark-active . nil) | |
a9f0a989 | 1370 | (buffer-undo-list . nil) |
e6512bcf RS |
1371 | (mode-name . "Fundamental") |
1372 | @dots{} | |
1373 | @group | |
177c0ea7 | 1374 | ;; @r{Next, non-built-in buffer-local variables.} |
f9f59935 | 1375 | ;; @r{This one is buffer-local and void:} |
e6512bcf | 1376 | foobar |
f9f59935 | 1377 | ;; @r{This one is buffer-local and nonvoid:} |
e6512bcf RS |
1378 | (bind-me . 69)) |
1379 | @end group | |
1380 | @end example | |
1381 | ||
1382 | Note that storing new values into the @sc{cdr}s of cons cells in this | |
f9f59935 | 1383 | list does @emph{not} change the buffer-local values of the variables. |
e6512bcf RS |
1384 | @end defun |
1385 | ||
1386 | @deffn Command kill-local-variable variable | |
1387 | This function deletes the buffer-local binding (if any) for | |
1388 | @var{variable} (a symbol) in the current buffer. As a result, the | |
f9f59935 RS |
1389 | default binding of @var{variable} becomes visible in this buffer. This |
1390 | typically results in a change in the value of @var{variable}, since the | |
1391 | default value is usually different from the buffer-local value just | |
1392 | eliminated. | |
e6512bcf | 1393 | |
f9f59935 RS |
1394 | If you kill the buffer-local binding of a variable that automatically |
1395 | becomes buffer-local when set, this makes the default value visible in | |
1396 | the current buffer. However, if you set the variable again, that will | |
1397 | once again create a buffer-local binding for it. | |
e6512bcf RS |
1398 | |
1399 | @code{kill-local-variable} returns @var{variable}. | |
f57ddf67 RS |
1400 | |
1401 | This function is a command because it is sometimes useful to kill one | |
1402 | buffer-local variable interactively, just as it is useful to create | |
1403 | buffer-local variables interactively. | |
e6512bcf RS |
1404 | @end deffn |
1405 | ||
1406 | @defun kill-all-local-variables | |
1407 | This function eliminates all the buffer-local variable bindings of the | |
1408 | current buffer except for variables marked as ``permanent''. As a | |
1409 | result, the buffer will see the default values of most variables. | |
1410 | ||
1411 | This function also resets certain other information pertaining to the | |
1412 | buffer: it sets the local keymap to @code{nil}, the syntax table to the | |
a9f0a989 RS |
1413 | value of @code{(standard-syntax-table)}, the case table to |
1414 | @code{(standard-case-table)}, and the abbrev table to the value of | |
1415 | @code{fundamental-mode-abbrev-table}. | |
e6512bcf | 1416 | |
f9f59935 | 1417 | The very first thing this function does is run the normal hook |
969fe9b5 | 1418 | @code{change-major-mode-hook} (see below). |
f9f59935 | 1419 | |
e6512bcf RS |
1420 | Every major mode command begins by calling this function, which has the |
1421 | effect of switching to Fundamental mode and erasing most of the effects | |
1422 | of the previous major mode. To ensure that this does its job, the | |
1423 | variables that major modes set should not be marked permanent. | |
1424 | ||
1425 | @code{kill-all-local-variables} returns @code{nil}. | |
1426 | @end defun | |
1427 | ||
969fe9b5 RS |
1428 | @defvar change-major-mode-hook |
1429 | The function @code{kill-all-local-variables} runs this normal hook | |
1430 | before it does anything else. This gives major modes a way to arrange | |
1431 | for something special to be done if the user switches to a different | |
bf614499 RS |
1432 | major mode. It is also useful for buffer-specific minor modes |
1433 | that should be forgotten if the user changes the major mode. | |
1434 | ||
1435 | For best results, make this variable buffer-local, so that it will | |
1436 | disappear after doing its job and will not interfere with the | |
969fe9b5 RS |
1437 | subsequent major mode. @xref{Hooks}. |
1438 | @end defvar | |
1439 | ||
e6512bcf RS |
1440 | @c Emacs 19 feature |
1441 | @cindex permanent local variable | |
f9f59935 RS |
1442 | A buffer-local variable is @dfn{permanent} if the variable name (a |
1443 | symbol) has a @code{permanent-local} property that is non-@code{nil}. | |
1444 | Permanent locals are appropriate for data pertaining to where the file | |
1445 | came from or how to save it, rather than with how to edit the contents. | |
e6512bcf RS |
1446 | |
1447 | @node Default Value | |
1448 | @subsection The Default Value of a Buffer-Local Variable | |
1449 | @cindex default value | |
1450 | ||
1451 | The global value of a variable with buffer-local bindings is also | |
1452 | called the @dfn{default} value, because it is the value that is in | |
969fe9b5 RS |
1453 | effect whenever neither the current buffer nor the selected frame has |
1454 | its own binding for the variable. | |
e6512bcf RS |
1455 | |
1456 | The functions @code{default-value} and @code{setq-default} access and | |
1457 | change a variable's default value regardless of whether the current | |
1458 | buffer has a buffer-local binding. For example, you could use | |
1459 | @code{setq-default} to change the default setting of | |
1460 | @code{paragraph-start} for most buffers; and this would work even when | |
f57ddf67 | 1461 | you are in a C or Lisp mode buffer that has a buffer-local value for |
e6512bcf RS |
1462 | this variable. |
1463 | ||
1464 | @c Emacs 19 feature | |
1465 | The special forms @code{defvar} and @code{defconst} also set the | |
f9f59935 | 1466 | default value (if they set the variable at all), rather than any |
969fe9b5 | 1467 | buffer-local or frame-local value. |
e6512bcf RS |
1468 | |
1469 | @defun default-value symbol | |
1470 | This function returns @var{symbol}'s default value. This is the value | |
969fe9b5 RS |
1471 | that is seen in buffers and frames that do not have their own values for |
1472 | this variable. If @var{symbol} is not buffer-local, this is equivalent | |
1473 | to @code{symbol-value} (@pxref{Accessing Variables}). | |
e6512bcf RS |
1474 | @end defun |
1475 | ||
1476 | @c Emacs 19 feature | |
f57ddf67 RS |
1477 | @defun default-boundp symbol |
1478 | The function @code{default-boundp} tells you whether @var{symbol}'s | |
e6512bcf RS |
1479 | default value is nonvoid. If @code{(default-boundp 'foo)} returns |
1480 | @code{nil}, then @code{(default-value 'foo)} would get an error. | |
1481 | ||
1482 | @code{default-boundp} is to @code{default-value} as @code{boundp} is to | |
1483 | @code{symbol-value}. | |
1484 | @end defun | |
1485 | ||
a9f0a989 RS |
1486 | @defspec setq-default [symbol form]@dots{} |
1487 | This special form gives each @var{symbol} a new default value, which is | |
1488 | the result of evaluating the corresponding @var{form}. It does not | |
1489 | evaluate @var{symbol}, but does evaluate @var{form}. The value of the | |
1490 | @code{setq-default} form is the value of the last @var{form}. | |
e6512bcf RS |
1491 | |
1492 | If a @var{symbol} is not buffer-local for the current buffer, and is not | |
1493 | marked automatically buffer-local, @code{setq-default} has the same | |
1494 | effect as @code{setq}. If @var{symbol} is buffer-local for the current | |
1495 | buffer, then this changes the value that other buffers will see (as long | |
1496 | as they don't have a buffer-local value), but not the value that the | |
1497 | current buffer sees. | |
1498 | ||
1499 | @example | |
1500 | @group | |
1501 | ;; @r{In buffer @samp{foo}:} | |
969fe9b5 RS |
1502 | (make-local-variable 'buffer-local) |
1503 | @result{} buffer-local | |
e6512bcf RS |
1504 | @end group |
1505 | @group | |
969fe9b5 | 1506 | (setq buffer-local 'value-in-foo) |
e6512bcf RS |
1507 | @result{} value-in-foo |
1508 | @end group | |
1509 | @group | |
969fe9b5 | 1510 | (setq-default buffer-local 'new-default) |
e6512bcf RS |
1511 | @result{} new-default |
1512 | @end group | |
1513 | @group | |
969fe9b5 | 1514 | buffer-local |
e6512bcf RS |
1515 | @result{} value-in-foo |
1516 | @end group | |
1517 | @group | |
969fe9b5 | 1518 | (default-value 'buffer-local) |
e6512bcf RS |
1519 | @result{} new-default |
1520 | @end group | |
1521 | ||
1522 | @group | |
1523 | ;; @r{In (the new) buffer @samp{bar}:} | |
969fe9b5 | 1524 | buffer-local |
e6512bcf RS |
1525 | @result{} new-default |
1526 | @end group | |
1527 | @group | |
969fe9b5 | 1528 | (default-value 'buffer-local) |
e6512bcf RS |
1529 | @result{} new-default |
1530 | @end group | |
1531 | @group | |
969fe9b5 | 1532 | (setq buffer-local 'another-default) |
e6512bcf RS |
1533 | @result{} another-default |
1534 | @end group | |
1535 | @group | |
969fe9b5 | 1536 | (default-value 'buffer-local) |
e6512bcf RS |
1537 | @result{} another-default |
1538 | @end group | |
1539 | ||
1540 | @group | |
1541 | ;; @r{Back in buffer @samp{foo}:} | |
969fe9b5 | 1542 | buffer-local |
e6512bcf | 1543 | @result{} value-in-foo |
969fe9b5 | 1544 | (default-value 'buffer-local) |
e6512bcf RS |
1545 | @result{} another-default |
1546 | @end group | |
1547 | @end example | |
1548 | @end defspec | |
1549 | ||
1550 | @defun set-default symbol value | |
1551 | This function is like @code{setq-default}, except that @var{symbol} is | |
f9f59935 | 1552 | an ordinary evaluated argument. |
e6512bcf RS |
1553 | |
1554 | @example | |
1555 | @group | |
1556 | (set-default (car '(a b c)) 23) | |
1557 | @result{} 23 | |
1558 | @end group | |
1559 | @group | |
1560 | (default-value 'a) | |
1561 | @result{} 23 | |
1562 | @end group | |
1563 | @end example | |
1564 | @end defun | |
969fe9b5 RS |
1565 | |
1566 | @node Frame-Local Variables | |
1567 | @section Frame-Local Variables | |
1568 | ||
1569 | Just as variables can have buffer-local bindings, they can also have | |
1570 | frame-local bindings. These bindings belong to one frame, and are in | |
1571 | effect when that frame is selected. Frame-local bindings are actually | |
1572 | frame parameters: you create a frame-local binding in a specific frame | |
1573 | by calling @code{modify-frame-parameters} and specifying the variable | |
1574 | name as the parameter name. | |
1575 | ||
1576 | To enable frame-local bindings for a certain variable, call the function | |
1577 | @code{make-variable-frame-local}. | |
1578 | ||
a9f0a989 | 1579 | @deffn Command make-variable-frame-local variable |
969fe9b5 RS |
1580 | Enable the use of frame-local bindings for @var{variable}. This does |
1581 | not in itself create any frame-local bindings for the variable; however, | |
1582 | if some frame already has a value for @var{variable} as a frame | |
1583 | parameter, that value automatically becomes a frame-local binding. | |
1584 | ||
f36e1798 LT |
1585 | If @var{variable} does not have a default value, then calling this |
1586 | command will give it a default value of @code{nil}. If @var{variable} | |
1587 | already has a default value, that value remains unchanged. | |
1588 | ||
a9f0a989 RS |
1589 | If the variable is terminal-local, this function signals an error, |
1590 | because such variables cannot have frame-local bindings as well. | |
1591 | @xref{Multiple Displays}. A few variables that are implemented | |
1592 | specially in Emacs can be (and usually are) buffer-local, but can never | |
1593 | be frame-local. | |
f36e1798 LT |
1594 | |
1595 | This command returns @var{variable}. | |
a9f0a989 | 1596 | @end deffn |
969fe9b5 RS |
1597 | |
1598 | Buffer-local bindings take precedence over frame-local bindings. Thus, | |
1599 | consider a variable @code{foo}: if the current buffer has a buffer-local | |
1600 | binding for @code{foo}, that binding is active; otherwise, if the | |
1601 | selected frame has a frame-local binding for @code{foo}, that binding is | |
1602 | active; otherwise, the default binding of @code{foo} is active. | |
1603 | ||
1604 | Here is an example. First we prepare a few bindings for @code{foo}: | |
1605 | ||
1606 | @example | |
1607 | (setq f1 (selected-frame)) | |
1608 | (make-variable-frame-local 'foo) | |
1609 | ||
1610 | ;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.} | |
1611 | (set-buffer (get-buffer-create "b1")) | |
1612 | (make-local-variable 'foo) | |
1613 | (setq foo '(b 1)) | |
1614 | ||
1615 | ;; @r{Make a frame-local binding for @code{foo} in a new frame.} | |
1616 | ;; @r{Store that frame in @code{f2}.} | |
1617 | (setq f2 (make-frame)) | |
1618 | (modify-frame-parameters f2 '((foo . (f 2)))) | |
1619 | @end example | |
1620 | ||
1621 | Now we examine @code{foo} in various contexts. Whenever the | |
1622 | buffer @samp{b1} is current, its buffer-local binding is in effect, | |
1623 | regardless of the selected frame: | |
1624 | ||
1625 | @example | |
1626 | (select-frame f1) | |
1627 | (set-buffer (get-buffer-create "b1")) | |
1628 | foo | |
1629 | @result{} (b 1) | |
1630 | ||
1631 | (select-frame f2) | |
1632 | (set-buffer (get-buffer-create "b1")) | |
1633 | foo | |
1634 | @result{} (b 1) | |
1635 | @end example | |
1636 | ||
1637 | @noindent | |
1638 | Otherwise, the frame gets a chance to provide the binding; when frame | |
1639 | @code{f2} is selected, its frame-local binding is in effect: | |
1640 | ||
1641 | @example | |
1642 | (select-frame f2) | |
1643 | (set-buffer (get-buffer "*scratch*")) | |
1644 | foo | |
1645 | @result{} (f 2) | |
1646 | @end example | |
1647 | ||
1648 | @noindent | |
1649 | When neither the current buffer nor the selected frame provides | |
1650 | a binding, the default binding is used: | |
1651 | ||
1652 | @example | |
1653 | (select-frame f1) | |
1654 | (set-buffer (get-buffer "*scratch*")) | |
1655 | foo | |
1656 | @result{} nil | |
1657 | @end example | |
1658 | ||
1659 | @noindent | |
1660 | When the active binding of a variable is a frame-local binding, setting | |
1661 | the variable changes that binding. You can observe the result with | |
1662 | @code{frame-parameters}: | |
1663 | ||
1664 | @example | |
1665 | (select-frame f2) | |
1666 | (set-buffer (get-buffer "*scratch*")) | |
1667 | (setq foo 'nobody) | |
1668 | (assq 'foo (frame-parameters f2)) | |
1669 | @result{} (foo . nobody) | |
1670 | @end example | |
1671 | ||
1672 | @node Future Local Variables | |
1673 | @section Possible Future Local Variables | |
1674 | ||
1675 | We have considered the idea of bindings that are local to a category | |
1676 | of frames---for example, all color frames, or all frames with dark | |
1677 | backgrounds. We have not implemented them because it is not clear that | |
1678 | this feature is really useful. You can get more or less the same | |
916a89db | 1679 | results by adding a function to @code{after-make-frame-functions}, set up to |
969fe9b5 RS |
1680 | define a particular frame parameter according to the appropriate |
1681 | conditions for each frame. | |
1682 | ||
1683 | It would also be possible to implement window-local bindings. We | |
1684 | don't know of many situations where they would be useful, and it seems | |
1685 | that indirect buffers (@pxref{Indirect Buffers}) with buffer-local | |
1686 | bindings offer a way to handle these situations more robustly. | |
1687 | ||
1688 | If sufficient application is found for either of these two kinds of | |
1689 | local bindings, we will provide it in a subsequent Emacs version. | |
1690 | ||
26236f6d GM |
1691 | @node Variable Aliases |
1692 | @section Variable Aliases | |
1693 | ||
1694 | It is sometimes useful to make two variables synonyms, so that both | |
1695 | variables always have the same value, and changing either one also | |
1696 | changes the other. Whenever you change the name of a | |
1697 | variable---either because you realize its old name was not well | |
1698 | chosen, or because its meaning has partly changed---it can be useful | |
1699 | to keep the old name as an @emph{alias} of the new one for | |
1700 | compatibility. You can do this with @code{defvaralias}. | |
1701 | ||
a1c07461 | 1702 | @defun defvaralias alias-var base-var &optional docstring |
26236f6d | 1703 | This function defines the symbol @var{alias-var} as a variable alias |
32ebbc3a | 1704 | for symbol @var{base-var}. This means that retrieving the value of |
26236f6d GM |
1705 | @var{alias-var} returns the value of @var{base-var}, and changing the |
1706 | value of @var{alias-var} changes the value of @var{base-var}. | |
32ebbc3a | 1707 | |
a1c07461 RS |
1708 | If the @var{docstring} argument is non-@code{nil}, it specifies the |
1709 | documentation for @var{alias-var}; otherwise, the alias gets the same | |
f36e1798 LT |
1710 | documentation as @var{base-var} has, if any, unless @var{base-var} is |
1711 | itself an alias, in which case @var{alias-var} gets the documentation | |
1712 | of the variable at the end of the chain of aliases. | |
1713 | ||
1714 | This function returns @var{base-var}. | |
3fdb4c50 | 1715 | @end defun |
26236f6d GM |
1716 | |
1717 | @defun indirect-variable variable | |
1718 | This function returns the variable at the end of the chain of aliases | |
1719 | of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is | |
1720 | not defined as an alias, the function returns @var{variable}. | |
571fd4d5 JH |
1721 | |
1722 | This function signals a @code{cyclic-variable-indirection} error if | |
1723 | there is a loop in the chain of symbols. | |
26236f6d GM |
1724 | @end defun |
1725 | ||
1726 | @example | |
1727 | (defvaralias 'foo 'bar) | |
1728 | (indirect-variable 'foo) | |
1729 | @result{} bar | |
1730 | (indirect-variable 'bar) | |
1731 | @result{} bar | |
1732 | (setq bar 2) | |
1733 | bar | |
1734 | @result{} 2 | |
f36e1798 | 1735 | @group |
26236f6d GM |
1736 | foo |
1737 | @result{} 2 | |
f36e1798 | 1738 | @end group |
26236f6d GM |
1739 | (setq foo 0) |
1740 | bar | |
1741 | @result{} 0 | |
1742 | foo | |
1743 | @result{} 0 | |
1744 | @end example | |
1745 | ||
b50c9214 RS |
1746 | @node File Local Variables |
1747 | @section File Local Variables | |
1748 | ||
1749 | This section describes the functions and variables that affect | |
f36e1798 LT |
1750 | processing of local variables lists in files. @xref{File variables, , |
1751 | Local Variables in Files, emacs, The GNU Emacs Manual}, for basic | |
1752 | information about file local variables. | |
b50c9214 RS |
1753 | |
1754 | @defopt enable-local-variables | |
1755 | This variable controls whether to process file local variables lists. A | |
1756 | value of @code{t} means process the local variables lists | |
1757 | unconditionally; @code{nil} means ignore them; anything else means ask | |
1758 | the user what to do for each file. The default value is @code{t}. | |
1759 | @end defopt | |
1760 | ||
f36e1798 | 1761 | @defun hack-local-variables &optional mode-only |
b50c9214 RS |
1762 | This function parses, and binds or evaluates as appropriate, any local |
1763 | variables specified by the contents of the current buffer. The variable | |
f36e1798 LT |
1764 | @code{enable-local-variables} has its effect here. However, this |
1765 | function does not look for the @samp{mode:} local variable in the | |
1766 | @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking | |
1767 | @code{enable-local-variables} into account. | |
1768 | ||
1769 | If the optional argument @var{mode-only} is non-@code{nil}, then all | |
1770 | this function does is return @code{t} if the @w{@samp{-*-}} line | |
1771 | specifies a mode and @code{nil} otherwise. It does not set the mode | |
1772 | nor any other file local variable. It does not check whether a mode | |
1773 | is specified in the local variables list at the end of the file. | |
b50c9214 RS |
1774 | @end defun |
1775 | ||
18684a3a RS |
1776 | If a file local variable list could specify a function that would |
1777 | be called later, or an expression that would be executed later, simply | |
b50c9214 RS |
1778 | visiting a file could take over your Emacs. To prevent this, Emacs |
1779 | takes care not to allow local variable lists to set such variables. | |
1780 | ||
7ed9159a | 1781 | For one thing, any variable whose name ends in @samp{-command}, |
f5d2538f | 1782 | @samp{-frame-alist}, @samp{-function}, @samp{-functions}, |
7ed9159a JY |
1783 | @samp{-hook}, @samp{-hooks}, @samp{-form}, @samp{-forms}, @samp{-map}, |
1784 | @samp{-map-alist}, @samp{-mode-alist}, @samp{-program}, or | |
1785 | @samp{-predicate} cannot be set in a local variable list. In general, | |
1786 | you should use such a name whenever it is appropriate for the | |
1787 | variable's meaning. The variables @samp{font-lock-keywords}, | |
1788 | @samp{font-lock-keywords-[0-9]}, and | |
1789 | @samp{font-lock-syntactic-keywords} cannot be set in a local variable | |
f36e1798 LT |
1790 | list, either. These rules can be overridden by giving the variable's |
1791 | name a non-@code{nil} @code{safe-local-variable} property. If one | |
1792 | gives it a @code{safe-local-variable} property of @code{t}, then one | |
1793 | can give the variable any file local value. One can also give any | |
1794 | symbol, including the above, a @code{safe-local-variable} property | |
1795 | that is a function taking exactly one argument. In that case, giving | |
1796 | a variable with that name a file local value is only allowed if the | |
1797 | function returns non-@code{nil} when called with that value as | |
1798 | argument. | |
b50c9214 RS |
1799 | |
1800 | In addition, any variable whose name has a non-@code{nil} | |
7ed9159a JY |
1801 | @code{risky-local-variable} property is also ignored. So are all |
1802 | variables listed in @code{ignored-local-variables}: | |
b50c9214 RS |
1803 | |
1804 | @defvar ignored-local-variables | |
1805 | This variable holds a list of variables that should not be | |
1806 | set by a file's local variables list. Any value specified | |
1807 | for one of these variables is ignored. | |
1808 | @end defvar | |
1809 | ||
f36e1798 LT |
1810 | @defun risky-local-variable-p sym &optional val |
1811 | If @var{val} is non-@code{nil}, returns non-@code{nil} if giving | |
1812 | @var{sym} a file local value of @var{val} would be risky, for any of | |
1813 | the reasons stated above. If @var{val} is @code{nil} or omitted, only | |
1814 | returns @code{nil} if @var{sym} can be safely assigned any file local | |
1815 | value whatsoever. | |
7ed9159a JY |
1816 | @end defun |
1817 | ||
b50c9214 RS |
1818 | The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs |
1819 | normally asks for confirmation before handling it. | |
969fe9b5 | 1820 | |
b50c9214 RS |
1821 | @defopt enable-local-eval |
1822 | This variable controls processing of @samp{Eval:} in local variables | |
1823 | lists in files being visited. A value of @code{t} means process them | |
1824 | unconditionally; @code{nil} means ignore them; anything else means ask | |
1825 | the user what to do for each file. The default value is @code{maybe}. | |
1826 | @end defopt | |
ab5796a9 | 1827 | |
2b99c69a RS |
1828 | Text properties are also potential loopholes, since their values |
1829 | could include functions to call. So Emacs discards all text | |
1830 | properties from string values specified in a file's local variables | |
1831 | list. | |
1832 | ||
74ab3aa3 LT |
1833 | @node Variables with Restricted Values |
1834 | @section Variables with Restricted Values | |
1835 | ||
1836 | Ordinary Lisp variables can be assigned any value that is a valid | |
1837 | Lisp object. However, certain Lisp variables are not defined in Lisp, | |
1838 | but in C. Most of these variables are defined in the C code using | |
1839 | @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on | |
1840 | any value. However, some variables are defined using | |
1841 | @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp | |
1842 | variables in C,, Writing Emacs Primitives}, in particular the | |
1843 | description of functions of the type @code{syms_of_@var{filename}}, | |
1844 | for a brief discussion of the C implementation. | |
1845 | ||
1846 | Variables of type @code{DEFVAR_BOOL} can only take on the values | |
1847 | @code{nil} or @code{t}. Attempting to assign them any other value | |
1848 | will set them to @code{t}: | |
1849 | ||
1850 | @example | |
1851 | (let ((display-hourglass 5)) | |
1852 | display-hourglass) | |
1853 | @result{} t | |
1854 | @end example | |
1855 | ||
1856 | @defvar byte-boolean-vars | |
1857 | This variable holds a list of all variables of type @code{DEFVAR_BOOL}. | |
1858 | @end defvar | |
1859 | ||
1860 | Variables of type @code{DEFVAR_INT} can only take on integer values. | |
1861 | Attempting to assign them any other value will result in an error: | |
1862 | ||
1863 | @example | |
1864 | (setq window-min-height 5.0) | |
1865 | @error{} Wrong type argument: integerp, 5.0 | |
1866 | @end example | |
1867 | ||
ab5796a9 MB |
1868 | @ignore |
1869 | arch-tag: 5ff62c44-2b51-47bb-99d4-fea5aeec5d3e | |
1870 | @end ignore |